Numerous processes have been employed in the past for forming sheets of paperboard into unitary, three dimensional container shapes such as those used for holding food products. Typically, molding or pressing apparatus is used to form the paperboard into the desired configuration. It is also known in the art to moisten and thereby soften the paperboard so as to render the same more flexible and thereby facilitate subsequent molding or pressing thereof. U.S. Pat. Nos. 2,997,927 and 3,675,692 are typical of prior art fiber board and paperboard forming processes which recognize the advantage of moistening the sheet stock prior to molding or pressing the same into the desired configuration.
Some container products, such as trays used for packaging food stuffs, benefit from the use of a plastic liner which conforms to the interior of the container and provides a two-way gas and moisture barrier to provide the dual benefits of preventing the paperboard from becoming damaged by the food stuff and preventing the food stuff from drying out or picking up odors from its environment. One method of applying the plastic liner to the container involves vacuum forming the liner into the interior of the container in a separate manufacturing operation after the paperboard container itself has been formed. This approach is undesirable from the standpoint of production economies since separate material handling and manufacturing steps are required. Another approach to manufacturing a lined container of the type described above involves first bonding a layer of plastic film to the sheet of paperboard and then forming the laminate by pressing, molding or drawing the same into the desired configuration.
This last mentioned approach to forming a plastic lined container poses a problem in the case of deep drawn containers, and particularly those made from paperboard stock in that delamination and/or blistering of the plastic film relative to the underlying paperboard frequently takes place. Additionally, the paperboard often undergoes some cracking or splitting upon forming. The physical mechanisms responsible for these defects have not been completely identified, however, it is believed that differences in physical properties of the paperboard and the plastic liner are largely responsible. Such differences include disparities in flexibility and coefficients of thermal expansion. For example, in the case of a laminate wherein the paperboard layer thereof has been pre-moistened prior to forming, the moistened paperboard is quite weak relative to the plastic layer of film; because the plastic film is intimately adhered to the paperboard, stresses imposed on the plastic film during forming are transmitted directly to the underlying paperboard thereby rupturing the latter.
The problems discussed above are exaggerated in the case of containers formed from a laminate of paperboard and plastic film which is deep drawn to a depth 1.375 inches or more because the relative pressures acting on and between the paperboard stock and the plastic film are substantially multiplied; in part, this is because portions of the laminate sheet are folded in order to provide compression of the sheet at the corners of the container. Compression of the laminate sheet at the corners of the container is considerably greater in those which are deep drawn in contrast to more shallow containers.
The known prior art process for forming a deep drawn laminated paperboard container involves an initial step of adding a selected amount of moisture to the paperboard layer of the laminate by immersing the laminate in a liquid bath and then winding the same into a roll; the tension applied to the laminate by the wind-up mechanism may be varied to squeeze excess moisture from the roll, thereby providing rough control of the amount of moisture finally imparted to the laminate. The above described moisture conditioning step is performed at least 24 hours prior to pressing the laminate into containers. The liquid employed in this prior art processing step comprised water at room temperature to which there had been added a wetting agent in the amount of 1 to 11/2% by weight. The total amount of moisture imparted to the paperboard was in the range of 6% to 15%. After being preconditioned by the moistening step described above, the laminate was formed in a die wherein the female portion thereof was heated to a temperature of 350.degree.-450.degree. F. and was adapted to contact the paperboard side of the laminate, while the male portion of the die was unheated and contacted the plastic film side of the laminate. The plastic film layer of the laminate was softened during the forming process by heat passing through the paperboard layer originating from the female die.
This prior art process is undesirable in several respects and is believed to be responsible for the various defects, such as delamination and blistering discussed in detail previously. For example, because the process step of applying moisture to the paperboard required an extended length of time to perform, this process step was carried out in a separate operation well in advance of the time that the laminate was actually formed into a container; thus, these separate manufacturing operations carried out at different points in time substantially added to manufacturing costs. Normally, a long length of the laminate was submerged in a relatively large tank filled with the moistening liquid. This necessitated the use of a relatively large tank which displaced valuable manufacturing floor space. Also, the costs associated with the need to add a wetting agent to the moistening liquid were rather substantial, particularly in light of the large quantities of moistening agent which were required to fill the tank. Finally, this prior art approach to premoistening the paperboard provided only rough control, at best, of the amount of moisture imparted to the paperboard, and precluded the possibility of continuous and/or contemporaneous variation of the moisture content of the paperboard immediately prior to forming thereof. The moisture content of the formed container could be controlled to a limited degree by increasing or decreasing the female die temperature in order to alter the amount of moisture in the paperboard which was converted to steam during the forming process, however, the latitude of control was extremely limited and varying the female die temperature had deleterious effects on the physical quality of the resulting container. This problem of controlling moisture in the paperboard was further complicated by the fact that it was necessary to also alter moisture content and the female die temperature to accomodate variations in the physical composition of the laminate.
It is therefore an important object of the present invention to provide an improved process for forming a plastic lined paperboard container which eliminates the problems of delamination or blistering of the liner relative to the paperboard as well as cracking or splitting of the paperboard.
Another object of the invention is to provide an improved process for forming a paperboard and plastic film laminate into a deep drawn container, wherein moisture is added to the paperboard immediately prior to forming thereof in a manner which allows the amount of moisture imparted to the paperboard to be contemporaneously varied to a desired degree.
A further object of the present invention is to provide an improved process as described immediately above which eliminates the necessity of adding moisture to the paperboard using a batch processing technique and allows moisture to be continuously added to the paperboard, as the laminate is delivered to a forming station.
A still further object of the invention, related to the foregoing object, is to eliminate the need for large holding tanks and quantities of moistening liquid, while also eliminating the need for a wetting agent heretofore added to the moistening liquid.
According to the present invention, a sheet laminate is deep drawn by pressing thereof into a three dimensional shape suitable for use as a container. The laminate comprises paperboard or other suitable stock having a layer of plastic, such as polyethylene, polyethylene terephthalate, a polyamide and the like bonded to one side thereof. A continuous sheet of the laminate is drawn in a continuous process from a moistening station where a selected amount of moisture is added to the paperboard substrate, to a forming station where the preconditioned laminate is formed into the container. Coating apparatus at the moistening station applies warm water to the bare (i.e. uncoated) side of the paperboard. The amount of liquid thusly applied may be conveniently altered to control the amount of moisture imparted to the paperboard, and the temperature of the water may be varied in order to preheat the laminate to a desired degree prior to forming thereof. The amount of additional moisture imparted to the laminate is controlled in part by the temperature of the liquid. The moistened, preheated laminate may be score and die cut as desired, and then delivered to a die press wherein the female portion of the die is heated to approximately 250.degree. F. Warming the moistening liquid increases the rate at which the paperboard absorbs the moisture, thereby eliminating the need for a wetting agent. Moisture content of the laminate, die temperatures, container quality, and the moisture content of the formed tray may be continuously monitored, compared and controlled in order to accomodate variations in the physical properties of the laminate. Indicia may be printed onto the laminate either before or after moisture is added thereto.